Apparatus for generating oscillations, which are also referred to as oscillators, emit, in known manner, periodic oscillations which exhibit a sinusoidal, rectangular or other such periodic waveform.
These output signals are defined by their amplitude and by their frequency f, whose reciprocal defines the periodic time T which is also referred to as the oscillatory period. Digital switching stages are particularly suited for generating rectangular output signals,
The so-called ring oscillator, which is known for example from the book "Introduction to VLSI systems", C. Mead, L. Conway, Addison-Wesley Publishing Company, 1980, represents one known embodiment of an oscillator using digital elements.
A ring oscillator is constructed in such a way that an odd number of inverters are connected one behind the other in series in a chain whereby the output of the last inverter is connected to the input of the first inverter.
Each of these inverters causes a time delay and the oscillatory period T of the output signal is determined by the number of inverters and the respective delay times.
The oscillatory period T itself amounts to twice the total delay time.
For various applications, but in particular in those cases in which the output signal of an oscillator is intended to be monitored by means of a phase controlled (PLL) circuit, it is advantageous if the oscillatory period T substantially corresponds to the total delay time of the inverter chain being used.